Quadrupole mass analyzers

Abstract: A quadrupole mass analyzer has a pair of heat sink plates (16, 17) and springs (18) that clamp ceramic holders (13, 14) that hold four rod electrodes (12) of a quadrupole unit of the analyzer. Heat generated dielectrically in the ceramic holders (13, 14) by a high frequency alternating electromagnetic field caused by an AC voltage applied to the four rod electrodes (12) is promptly transferred to the heat sink plates (16, 17), thereby preventing displacement of the four rod electrodes (12) or loss of symmetry thereof when a mass of ions is analyzed.

Description: This invention relates to quadrupole mass analyzers and to quadrupole units thereof. Fig. 5 of the accompanying drawings shows a previously proposed quadrupole mass analyzer which includes a quadrupole unit 40, an ion detector 46 positioned at an exit of the quadrupole unit 40, and a driver circuit 50. The quadrupole unit 40 is composed of four rod electrodes 41, 42, 43, 44 positioned parallel to and symmetrically around a z axis. The driver circuit 50 applies both a direct current (DC) voltage U and a high frequency alternating current (AC) voltage V.cos( ω .t) simultaneously between a pair 42, 44 of the electrodes that are positioned along an x axis and the other pair of electrodes 41 and 43, which are positioned along the y axis. When ions are introduced into the centre of an end of the quadrupole unit 40 while the DC/AC voltage is applied to the four rod electrodes 41, 42, 43, 44, only ions 45 having a specific mass can pass through the quadrupole unit 40: other ions 47 disperse before reaching the ion detector 46. Since the specific mass of the ions that can pass through the quadrupole unit 40 is determined by the DC voltage U and the high frequency AC voltage V, the mass of ions 45 detected by the ion detector 46 can be scanned by changing the values of the voltages U and V with a certain correlation between them. The four rod electrodes 41, 42, 43, 44 of the quadrupole unit 40 must be positioned precisely symmetrically around the z axis to perform a correct mass analysis. Thus, it has been proposed that the four rod electrodes 41, 42, 43, 44 be securely held by a pair of ceramic holders 48 and 49 at respective ends of the rod electrodes 41, 42, 43, 44, as shown in Figs. 6 and 7, to prevent displacement of the rod electrodes within the quadrupole unit 40. Then, the quadrupole unit 40 is inserted into a cylindrical case 52, as shown in Fig. 6, or placed on a base plate 53, as shown in Fig. 7, such that the unit 40 is correctly aligned with an ion entrance and with the ion detector 46 (not shown in Figs. 6 and 7). A quadrupole mass analyzer according to the first (pre-characterising) part of claim 1 is disclosed in US Patent No. US-A-4 032 782. When the high frequency AC voltage is applied to the four rod electrodes 41, 42, 43, 44 as described above in order to perform a mass analysis, the ceramic holders 48 and 49 are subjected to a high frequency alternating electromagnetic field and heat is generated in the ceramic holders 48 and 49 due to the dielectric heating effect. As the temperature rises due to the dielectric heating, the ceramic holders 48 and 49 expand and sometimes distort, resulting in a displacement or loss of symmetry of the rod electrodes 41, 42, 43, 44. In the quadrupole mass analyzers as shown in Figs. 6 or 7, the heat generated in the ceramic holders 48 and 49 can hardly escape because the area of contact between the cylindrical case 52 and the ceramic holders 48, 49, or between the base plate 53 and the holders 48, 49, is small and the cylindrical case 52 and the base plate 53 are made of stainless steel. According to the invention there is provided a quadrupole mass analyzer comprising: four rod electrodes positioned parallel with and symmetrically around a central axis; a pair of electrically non-conductive holders for holding the four rod electrodes at respective ends of the four rod electrodes; and at least one pair of heat sink plates and spring means for clamping the electrically non-conductive holders. Several preferred features of the invention are set out in the dependent claims. When the quadrupole unit is used in a mass analysis operation, the DC/AC voltage is applied to the four rod electrodes as described above, which produces a high frequency alternating electromagnetic field around the rod electrodes and causes dielectric heating in the non-conductive holders. The heat generated in the non-conductive holders is promptly transferred to the pair or pairs of heat sink plates. This prevents a marked temperature rise in the non-conductive holders, and displacement of the rod electrodes within the quadrupole unit is minimised, which ensures a correct mass analysis for a long time. It is preferable to match the shape of contacting faces of the non-conductive holders and the heat sink plates to increase the area of contact of the holders and plates. It is further preferable to make the contacting faces flat and parallel in order not to exert uneven forces on the non-conductive holders or the quadrupole unit. The heat sink plates can be made of copper, aluminium, steel or other metals having good thermal conductivity. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a quadrupole unit according to a first embodiment of the invention; Fig. 2 is a side view of the quadrupole unit placed in and contacting a heat conductive case; Fig. 3A is a front view of a second embodiment of the invention, which is equipped with a fluid cooling system: Fig. 3B is a side view of a third embodiment using Peltier heat pump units; Fig. 4 is a perspective view of a fourth embodiment, which has separate heat sink plates; Fig. 5 is an explanatory view of a quadrupole unit and the movement of ions; Fig. 6 is a perspective view of a previously proposed quadrupole unit using a cylindrical case; and Fig. 7 is a perspective view of another previously proposed quadrupole unit using a base plate. A first embodiment of the present invention will now be described with reference to Fig. 1. A quadrupole unit 11 is made up of four rod electrodes 12 positioned symmetrically around and parallel with a central axis and held or fixed by a pair of electrically non-conductive (preferably ceramic) holders 13 and 14 at their respective ends. The holders 13 and 14 are of octagonal shape, and have flat faces 13a, 13b, 14a and 14b at the top and at the bottom (or at the left and at the right). The holders 13, 14 are then clamped by a pair of copper or aluminium heat sink plates 16, 17 with four springs 18, whereby the holders 13, 14 and the heat sink plates 16, 17 contact one another over sufficiently broad areas and no skewing force is exerted on the quadrupole unit 11 because the holders 13, 14 can slide on the flat surfaces of the heat sink plates 16, 17. The heat generated in the rod electrodes 12 and the holders 13, 14 by dielectric heating during a mass analysis operation is transferred to the heat sink plates 16, 17 via the broad contacting faces whereby severe temperature rise of the holders 13, 14 is prevented. The quadrupole unit 11 clamped by the heat sink plates 16, 17 is then inserted in an appropriate case 52 as shown in Fig. 6 or placed on a base plate 53 as shown in Fig. 7. Since the position of the case 52 or the base plate 53 is fixed with respect to the ion entrance and the ion detector (not shown) taking account of the dimensions of the heat sink plates 16, 17, the quadrupole unit 11 aligns with the ion entrance and the ion detector so that ions coming through the ion entrance enter on the central axis of the quadrupole unit 11. When the quadrupole unit is installed in a mass analyzer, it is preferable to set the heat sink plates 16, 17 (or at least one of them) so as to contact a wall of a metal case 19 of the mass analyzer, as shown in Fig. 2. The heat transferred from the ceramic holders 13, 14 to the heat sink plates 16, 17 is then dissipated through the contacting faces to the metal case 19. Since the metal case 19 of a mass analyzer normally has a large heat capacity, the heat is effectively drawn out of the heat sink plates 16, 17, which further prevents the temperature rise of the ceramic holders 13, 14 and the quadrupole unit. In a second embodiment of the invention shown in Fig. 3A, a fluid cooling system is mounted on the heat sink plates 16, 17. The fluid cooling system includes a bottom heat exchanger 21, a top heat exchanger 23 and tubes 20, 22 and 24 for the flow of coolant through them. It is preferable for the coolant to flow from the bottom to the top. Fig. 3B shows a third embodiment of the invention in which Peltier heat pump units 25 are used to actively draw heat from the heat sink plates 16, 17 and actively give the heat to the metal case 19 of the mass analyzer. In Figs. 2 and 3B, a numeral 19a denotes a mass filter section; a numeral 19b denotes an ion source section; a number 19c denotes a vacuum pump section; and small arrows indicate the flow of heat. A fourth embodiment of the invention is shown in Fig. 4, in which a quadrupole unit 11 is clamped by two pairs of heat sink plates 31, 32 and 33, 34. The separate heat sink plates 31, 32, 33, 34 of the present embodiment are further resistant or resilient to misalignment of the ceramic holders 13, 14: that is, they do not exert a skewing force on the four rod electrodes 12 when the contacting faces 13a, 13b, 14a, 14b of the quadrupole holders 13, 14 are uneven. It is also possible in the present embodiment to use the fluid cooling system of Fig. 3A or the Peltier heat pump units of Fig. 3B.

Claims: A quadrupole mass analyzer comprising: four rod electrodes (11) positioned parallel with and symmetrically around a central axis; and a pair of electrically non-conductive holders (13, 14) for holding the four rod electrodes (11) at respective ends of the four rod electrodes; characterised by at least one pair of heat sink plates (16/17; 31/32, 33/34) and spring means (18) for clamping the electrically non-conductive holders (13, 14). A quadrupole mass analyzer according to claim 1, comprising one said pair of heat sink plates (16/17) and spring means (18) that clamps both of the non-conductive holders (13, 14). A quadrupole mass analyzer according to claim 1, comprising two said pairs of heat sink plates (31/32, 33/34) and respective spring means (18), each for clamping a respective one of the non-conductive holders (13, 14). A quadrupole mass analyzer according to claim 1, claim 2 or claim 3, wherein two parallel planar faces (13a, 13b, 14a, 14b) are formed on each of the non-conductive holders (13, 14) and surfaces of the heat sink plates (16/17; 31/32, 33/34) for contacting the non-conductive holders are flat. A quadrupole mass analyzer according to any one of the preceding claims, wherein at least one of the heat sink plates (16/17; 31/32, 33/34) is set to contact a wall of a metal case (19) of the quadrupole mass analyzer. A quadrupole mass analyzer according to claim 5, wherein at least one Peltier heat pump unit (25) is arranged to actively draw heat from at least one of the heat sink plates (16/17; 31/32, 33/34) to the wall of the metal case (19) of the quadrupole mass analyzer. A quadrupole mass analyzer according to any one of the preceding claims, wherein a fluid heat exchanger (21, 23) is provided on an outer surface of at least one of the heat sink plates (16/17; 31/32, 33/34). A quadrupole mass analyzer according to any one of claims 1 to 7, wherein the heat sink plates (16/17; 31/32, 33/34) are made of copper. A quadrupole mass analyzer according to any one of claims 1 to 7, wherein the heat sink plates (16/17; 31/32, 33/34) are made of aluminium.

Inventors: WAKI HIROAKI, WAKI, HIROAKI, WAKI, HIROAKI, C/O SHIMADZU CORPORATION
Applicants: SHIMADZU CORP, SHIMADZU CORPORATION
Application year: 1994
Main classification: H01J49
IPC codes: H01J49